| Post Info | TOPIC: Novel Double JET Blade VAWT |
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turnymf
Veteran Member
Posts: 39
Date: Mar 9, 2010
| RE: Novel Double JET Blade VAWT |
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| sjh7132 wrote:
There was no assumption of a round area.
Not sure about that one Steve Every model I have seen has been a disk relative to the wind direction For me it's a math figure relating to hawt props A hawt prop develops useable torque for the full 360 of rotation unlike vawts So betz limits, for me, is for Hawts For vawts I think it may be possible to have a second crack at getting energy from the wind with the back wing, some sort of wake capture the way a fly wing works http://www.scientificamerican.com/article.cfm?id=fly-like-a-flyWho knows? Thats my current thinking with the miller I'm playing with If I was off grid I would have hawts for sure, even with a 59% limit , because each wing is flying all the time But I'm on grid and I like to play with vawts, O..... and I live in suburbia and can live with a lower figure It's all fun Cheers |
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sjh7132
Guru
Posts: 1252
Date: Mar 8, 2010
| Novel Double JET Blade VAWT |
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| Bernd wrote:
Your thinking that we have to see a VAWT like a cylinder, related to the Betz limit, is very interessting. May be you are right.
Now I'm not sure. I just followed through the derivation Betz did and the only assumptions he made were that the air enters and exits in the same direction perpendicular to the surface. There was no assumption of a round area. And drag never factored into it. He just used conservation of mass and energy. It could be the cylindrical shape of the VAWT blows that assumption, but that only makes things worse for us. Interestingly, my method using drag and Betz's without it come up with the same answer for the ideal amount of speed to remove form the air. This is independent of the Cd of the object. The power output calculations don't quite agree so maybe I made a bad assumption somewhere. I'm not sure that size matters. Because for a given shape there is a Cd (for a Reynolds number range) Then you just use the equation F = 1/2 * density * Cd * area * airspeed ^2 to find the force. So force scales linearly with area, as we know power does most of the time. -- Edited by sjh7132 on Monday 8th of March 2010 11:25:31 PM
Steve
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Bernd
Senior Member
Posts: 233
Date: Mar 8, 2010
| Novel Double JET Blade VAWT |
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Steve if you are right, not only the shape of turbine, also the size must be important for the limit it can reach.
I think this because of the fact that on a very big turbine the wind couldn't dodge as easy as it can slip by at a very small turbine... :)
At a very big turbine the wind into the center gets much more pressure to pass the turbine than at a small turbine, where the wind can easy slip by the turbine instead of passing it through the repeller.
Your thinking that we have to see a VAWT like a cylinder, related to the Betz limit, is very interessting. May be you are right.
Bernd
-- Edited by Bernd on Monday 8th of March 2010 10:55:31 PM
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sjh7132
Guru
Posts: 1252
Date: Mar 8, 2010
| Novel Double JET Blade VAWT |
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| Just in case, I got too long winded in my last post, let me make my point in only a few lines. 1) Stone I agree with you 100%, the derivation was done on a flat circle, and best applies to HAWTs, other shaped turbines will have different limits. 2) Unfortunately for us, the flat round plate is the BEST shape for gathering power because it generates the most force per air speed. (see Drag on wikipedia). The flat circle can be considered a very short cylinder with a Cd of 1.15. You'll notice all other shapes have less drag, and thus less power. 3) Bottom line is I THINK this means VAWTs (which are basically vertical cylinders) have a lower limit than Betz. I'm much surer that they can't possibly have a higher limit than Betz. Thinking about this also shows me why 2d sims can seem to break the Betz limit. It's because the air can't slip by in the 3rd dimension. -- Edited by sjh7132 on Monday 8th of March 2010 10:23:46 PM
Steve
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Bernd
Senior Member
Posts: 233
Date: Mar 8, 2010
| Novel Double JET Blade VAWT |
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Betz itself doesn't "make" this limit, the nature of physics and atmosphere did it, Betz only find it.
We can discus if the limit is exactly the 59,3% value or a half % more or less, but not if its significant around.
It doesn't matter if you slow down the wind using a turbine (round or square) or a flat plate or drag construction or or or or....
The following wind will doge this "obstacle" !!!
You can hope it does not when you use a square turbine, but in reallity it will do.
What do you think is the new limit using a non disk like rotor, 70 %.. 80%...90%....??
At 100% the wind stands still.
Steve was faster than i, what he wrote is exactly how Betz limit is defined.
Its the "power peak" of harvesting windenergy, of harvesting windspeed.
Above this point the wind gets very slow, passing the turbine, so that new wind could not follow fast enough.
Above this point the wind is rather going around the obstacle instead of flow through the turbine. Thats the point !!!
"... A physical law has to be prooved. ..."
This limit is prooved every day on this earth, since Betz has written it.
Prooved from thousands of manufactors who wanted to build a efficient windturbine.
Until now no one has ever reached the limit.... let alone beyond. :)
The best Turbines are around 50% (acording to what the manufacturers specified..)
No one will ever be able to reach the limit because of the losses.
Facts are facts
Bernd
-- Edited by Bernd on Monday 8th of March 2010 10:54:08 PM
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sjh7132
Guru
Posts: 1252
Date: Mar 8, 2010
| Novel Double JET Blade VAWT |
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| stonebrain wrote:
Dear sjh,
I don't try to cheat laws of physics.
Physical laws are based on prooves.
Betz law is based on derivations based on aerflow through a propeller like rotor,that is, a thin disklike rotor. ..... facts are facts.
cheers
Stone, Sorry, I was again too aggressive. Yes, the derivation was based on a circle. (no blades involved.) Imagine we have a magic circle that changes the speed of the air going through it. I can calculate the power the circle gathers by multiplying the force of the wind pushing on the area of the circle * the speed of the air going through it. For example 10n * 5 m/s = 50 nm/s or 50 watts Lets say the wind is 10m/s, and lets set the magic circle so that it lets all the air go through at 10 m/s, same as the wind. In this case there will be no force pushed on the circle since the speed doesn't change, but we have a 10m/s speed. 0 * 10m/s = 0 watts. Now lets look at the other extreme. Lets make the circle stop the air dead. In this configuration there will be a large force from the wind pushing on the solid circle because it has to change directions and go around the circle, but there is no velocity through the circle. F * 0 m/s = 0 watts. Now, lets slowly let the speed of the air increase through the circle. At first we will have small V and large F, and small V * large F is small (non-zero) power. As we increase the V, the force will go down, and the V will go up. Until we hit some Max. At that point F * V is the maximum power we can get. Then the V becomes larger, but the F becomes so small that the power goes down again. So at some magic V and F there is a maximum power we can extract. Is it so hard to imagine that if I were to run through the equations, I'd find that maximum power is 59% of the wind power? I see your argument that Betz used a round flat surface for his derivation. But what about this thought experiment is dependent on the shape? I could use a flat square, etc. What affect does the shape have on the experiment? Well, I guess it would be the force for a given air speed change. (How hard it is to get the extra air to go around.) A round ball will obviously have less force since the air can flow around easier. So the limit for a magic ball turbine would be less than Betz. So I'd argue that given the flat circle has the most drag for a given air speed change, the Betz limit based on this shape, is a best case power limit. (The given part of this statement is the one that will be hard to prove, but I challenge you to find a shape that generates more drag per area than a simple flat circle.) I'll hold off on the 2 prop experiment until I see if we can agree about what's said here first. -- Edited by sjh7132 on Monday 8th of March 2010 09:23:52 PM
Steve
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stonebrain
Veteran Member
Posts: 78
Date: Mar 8, 2010
| RE: Novel Double JET Blade VAWT |
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Dear sjh,
I don't try to cheat laws of physics.
Physical laws are based on prooves.
Betz law is based on derivations based on aerflow through a propeller like rotor,that is, a thin disklike rotor.
I don't say that what you're saying is wrong,I'm just talking about what is prooved by mr Betz.
I just invite you to run through the articles you can find about Betz derivation I'm quite sure they are allways talking about a thin disklike rotor.
I don't want to give the impression that I want to second irrealistic claims on rotor efficiencies.
you say "If you have 2 inefficient propellers, then you might get a gain from 2 in a row. But if the first is already pushing Betz, then the second will definitely slow the first. The sum of the two will not exceed the 59%."This makes sense,but this makes not a physical law.A physical law has to be prooved.
facts are facts.
cheers
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sjh7132
Guru
Posts: 1252
Date: Mar 8, 2010
| RE: Novel Double JET Blade VAWT |
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| stonebrain wrote:
Bernd,I don't agree with you,but I'm not an alchemist.
Suppose,you put two propellers one behind another.
The second one allthough it's in the windshade of the other still can generate some(small amount)
electricity.This is reality you can observe.
Well,if you consider the two propellers as one,you exceed the betzlimit.
This is no magic at all.
Consider the basic conditions for wich the Betz limit was difined,to say a disklike rotor,
as any mathematical derivation,it only apply if you respect the condition for wich it was defined.
This is matematics.
cheers
Sorry but Bernd is 100% correct on this one. You can't cheat physics. If you put 2 propellers behind each other, the second creates a little back pressure that slows the front one down. If you have 2 inefficient propellers, then you might get a gain from 2 in a row. But if the first is already pushing Betz, then the second will definitely slow the first. The sum of the two will not exceed the 59%. Betz was derived with a perfect circular area that magically extracted energy from the air. He had to pick a shape to calculate area. It doesn't matter if it's a square, round or some other shape. It doesn't matter if it has thickness as long as air does not enter the volume from the side. (If it does then the area number is wrong.)
Steve
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stonebrain
Veteran Member
Posts: 78
Date: Mar 8, 2010
| RE: Novel Double JET Blade VAWT |
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Bernd,I don't agree with you,but I'm not an alchemist.
Suppose,you put two propellers one behind another.
The second one allthough it's in the windshade of the other still can generate some(small amount)
electricity.This is reality you can observe.
Well,if you consider the two propellers as one,you exceed the betzlimit.
This is no magic at all.
Consider the basic conditions for wich the Betz limit was difined,to say a disklike rotor,
as any mathematical derivation,it only apply if you respect the condition for wich it was defined.
This is matematics.
cheers
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Bernd
Senior Member
Posts: 233
Date: Mar 7, 2010
| Novel Double JET Blade VAWT |
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"... AFAIK The betz limit goes for a disklike rotor,like a propeller...."
No thats not true. Its for all windturbines and for all systems which take energy from the wind. Regardless how they are build.
If you slow down the wind, regardless which kind of turbine you use for doing this, the "new wind" couldn't reach the rotor, because the slowed down wind is preventing this.
59% is an theoretical optimisted value. It requires that the blades have no drag and no friction and no other losses and can compleytly transfer the whole windenergy to mechanical energy without transfering some of the windenergy to heat energy (air friction, other losses and so on)
So no one will ever be able to get to this limit, exept for him this law does not apply.
Some times for me it seems like in middle ages, as some alchemists want to make gold from plumbum oder mercury...
Bernd
-- Edited by Bernd on Sunday 7th of March 2010 10:24:33 PM
-- Edited by Bernd on Monday 8th of March 2010 10:32:59 AM
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stonebrain
Veteran Member
Posts: 78
Date: Mar 7, 2010
| RE: Novel Double JET Blade VAWT |
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AFAIK The betz limit goes for a disklike rotor,like a propeller.
So,who knows,it's not exactly the same for a hawt or any cilinder like rotor.
It's up to the mathematical geniuses know...
cheers,
stonebrain
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keithturtle
Senior Member
Posts: 174
Date: Mar 6, 2010
| RE: Novel Double JET Blade VAWT |
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| Whether or not George beats Betz is easy enough to quantify: power productiion in a given wind for an equal swept area. His Darrius / savonis concept is very similar to Hi-energy's production model http://www.hi-energy.com.tw/selection-6-english.htm#2My own expectations are to have the VAWT producing about 15% less power than a HAWT, swept area and all other variables being equal. I'll be happy with that. Time will tell Turtle
Soli Deo Gloria |
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sjh7132
Guru
Posts: 1252
Date: Mar 4, 2010
| RE: Novel Double JET Blade VAWT |
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| Tonchev wrote:
1. According to Betz - the energy of passing VAWT wind takes in account. In reality the energy of wind passing arround VAWT rotor have to take in account, as well.
I have been trying to think of ways to use the air passing around a turbine, but haven't been able to think of any. If you put fins up to funnel air into the turbine, then those fins count at part of the turbine and turbine size. Do you have some technique in mind, that makes use of the air outside of the area of the turbine?
Steve
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Bernd
Senior Member
Posts: 233
Date: Mar 4, 2010
| Novel Double JET Blade VAWT |
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The Betz law is a general law and so it musn't note thinks like how a turbine is build.
For the Betz law it's completly regardless how the turbine is build.
Solely the value of 59% is interessting.
Like Steve wrote before, if you go to take more than 59% of the wind, the wind slows so much down that
new air couldn't flow to the turbine blades, because of the "slowed down wind" is an obstacle for the new wind.
This applies to all Turbines, regardless how they are build. Its a general problem.
At 100% the wind isn't wind, it's just air, standing still.
What is wind energy ? It's the energy of the wind speed. No windspeed, no energy.
Each Windturbine, wich takes power from the wind, slows the wind down,
but it couldn't reduces it so much down that
the wind would be standing still. That makes no sense.
So you can see see it's not important how the turbine ist build, if drag, oder lift or horizontal or VAWT or or or....
if the Turbine goes to reduces the windspeed to much (and thats the point), the wind will dodge the turbine.
No chance, that are physical limits.
Bernd
-- Edited by Bernd on Thursday 4th of March 2010 12:33:23 PM
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Tonchev
Newbie
Posts: 1
Date: Mar 4, 2010
| RE: Novel Double JET Blade VAWT |
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| 1. According to Betz - the energy of passing VAWT wind takes in account. In reality the energy of wind passing arround VAWT rotor have to take in account, as well. 2. The real wind crossing and pasing arround a VAWT rotor is verry different than 2D rotor plane modelof Betz . 3. Every blade tip create energy losses. End less blades reduce tip losses significantly 4. Vortex effect in and out of VAWT rotor not includes in Betz model .... ..... N. Because of p.1 to p.N-1 Betz limit is not a real limit for 3D rotors See new invrnted ROTOJET turbine technology at www.tonchev.org |
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sjh7132
Guru
Posts: 1252
Date: Jan 19, 2010
| RE: Novel Double JET Blade VAWT |
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| turnymf wrote:
I am not sure it deserves the flaming it just received, at least not until someone can come up with some figures
miscbeep wrote
"theoretical efficiency is as high as above 80%"
This I think unlikely, as are a lot of claims on the internet regarding efficiency, particularly if money is involved
Even so, I for one like it
The Betz law, which is just based on physics (conservation of mass, momentum, and energy) states that 59% is the efficiency a perfect turbine would have. When you take the energy out of the air, it slows down. If you take too much energy, the slow air is in the way of new air coming in. There is no way to design around that limit. If someone has a company and is trying to sell turbines, or get investors and they claim 80% efficiency, I think they deserved to get flamed. If they claimed 58%, I'd have my doubts, but I would wait to see data.
Steve
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turnymf
Veteran Member
Posts: 39
Date: Jan 19, 2010
| RE: Novel Double JET Blade VAWT |
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| Caleb wrote:
It looks like a sleek version of the Miller Rotor.
I agree with Caleb I am not sure it deserves the flaming it just recieved, at least not until someone can come up with some figures miscbeep wrote "theoretical efficiency is as high as above 80%" This I think unlikely, as are a lot of claims on the internet regarding efficiency, particularly if money is involved Even so, I for one like it I am thinking about building a model but stacking three (120 dg apart)to reduce the jerking that I would think 1 would produce |
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Bernd
Senior Member
Posts: 233
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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And i wonder what would be happen, if the guy would place the fan center to the rotor ?
I have a new idea. Let us put 3 windturbines behind each other, so we can use the same wind 3 times too. :)
I guess the third Tubine would generate the most energy, because the wind, coming to the third turbine,
knows turbines already and so works much more efficient. :)
Bernd
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sjh7132
Guru
Posts: 1252
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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| Bernd wrote:
I'll wait for the version with 3 blades, that uses the same wind at least three times. :)
So it will have 3 x 40 % = 120 %
Oh is that how they do it? Why didn't I think of that? :-)
Steve
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Bernd
Senior Member
Posts: 233
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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I'll wait for the version with 3 blades, that uses the same wind at least three times. :)
So it will have 3 x 40 % = 120 %
Bernd
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Caleb
Senior Member
Posts: 473
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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It looks like a sleek version of the Miller Rotor.
__________________
- Bryan
Mechanical Engineer turned stay-at-home dad. |
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BerndsHelper
Member
Posts: 9
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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and I think it's not even aerodynamic enaugh to have an efficiency over the one of the Lenz/C/Darrieus-rotors...
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sjh7132
Guru
Posts: 1252
Date: Jan 18, 2010
| RE: Novel Double JET Blade VAWT |
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Unfortunately they claim to get 80% efficiency, claiming that breaking the Betz limit is not a problem. So at this point I don't believe anything they say.
__________________
Steve
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turnymf
Veteran Member
Posts: 39
Date: Jan 18, 2010
| Novel Double JET Blade VAWT |
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